This invention relates to low-expansion nickel-iron alloys and is particularly concerned with the provision of castings of such alloys, which in the as-cast form have high strength and low thermal expansion characteristics at service temperatures.
It is known that certain nickel-iron alloys have a remarkably low coefficient of thermal expansion such as, for example, an alloy of 36% nickel and 64% iron known under the trade name "Invar" which has a coefficient of thermal expansion approaching zero over the temperature range 0.degree. to around 200.degree.C. A major problem with low-expansion nickel-iron alloys is their low strength. One method by which the strength of such alloys can be increased is by addition of elements such as aluminum, titanium or niobium, and a subsequent ageing treatment.
Titanium has generally been added in amount of between 0.75% and 2.5% by weight to increase the strength of wrought alloys, and we have found that to achieve an increase in strength to comparable levels in cast alloys requires the addition of rather more titanium, that is, between 1.5 and 5% titanium by weight. However, as is well known, the increase in strength resulting from titanium additions is achieved at the expense of the low coefficient of thermal expansion which is increased in proportion to the increasing titanium content of the alloy.
Surprisingly we have now found that an optimum balance between high strength and a low coefficient of thermal expansion at temperatures in the range of 20.degree. to 300.degree.C can be achieved in a cast and aged nickel-iron alloy strengthened with titanium, by correlating the nickel and titanium contents and optional cobalt and niobium contents according to a specific relationship.
It is an object of the present invention to provide improved age-hardened iron-nickel-titanium alloys with predetermined low expansion characteristics which have high mechanical strength.
It is another object of this invention to provide low expansion iron-nickel-titanium alloys which may contain cobalt, and/or niobium and in which concentrations of nickel, cobalt, titanium and niobium are controlled and correlated.
It is a further object to provide low expansion iron-nickel-titanium alloys having in the as-cast and aged condition a linear thermal expansion coefficient between 20.degree. and 300.degree.C of less than about 6 .times. 10.sup.-.sup.6 /.degree.C and preferably less than about 5 .times. 10.sup.-.sup.6 /.degree.C, and a 0.2% proof stress at 20.degree.C higher than about 350 N/mm.sup.2.
A still further object is to provide an alloy having low expansivity and high strength at working temperatures which can be cast directly into intricately-shaped castings with good surface properties.
The invention also contemplates providing structural components of machinery, for example turbine shafts and blades, in which close dimensional tolerance must be maintained at temperatures up to about 500.degree.C, made of cast, age-hardenable alloys.
Other objects and advantages will become apparent from the accompanying drawings and the following description and examples.